Two examples of application of the dynamic power-law index method to the bursts. The upper panels show the temporal evolution of the flux, while the lower panels give power-law indices. Left panels: aPRE burst from 4U 1728–34 (MJD 51 238.79155), which shows a nearly power-law dependence of flux on time; the power-law index α varies smoothly (panel c)). Right panels: a non-PRE burst from 4U 1636–536 (MJD 52 290.14847), whose temporal evolution does not follow a power law with the corresponding power-law index shown in panel d). In the lower panels the black, blue, and green areas represent the 1σ confidence levels of the power-law indices of the individual 3, 7, and 11 point fits, respectively. The power-law indices are shown for the entire burst, but we study only the cooling tails. Also in the lower panels the red area shows the typical α-regime if the cooling is dominated by electrons, and the cyan area corresponds to ion-dominated cooling (see text).